Microscopic devices for moving toward specific, predetermined targets to isolate and detect those targets could be invaluable for a variety of applications, such as environmental monitoring, medical diagnostic, or therapeutic applications. However, many of the components required by such microscopic devices are too small or overly complex to be fabricated using conventional lithographic tools. Manipulation of matter at a nanoscale level is accordingly highly advantageous.
Self-assembly reactions provide one route for constructing 1-, 2-, and 3-dimensional objects in this size regime. For example, covalent bonds between alkanethiols and gold substrates have been used to pattern surfaces in 2 dimensions, surface affinity properties have been used to self-assemble colloidal photonic crystals in 2 and 3 dimensions, and complementary interactions between DNA base pairs have been used to assemble nanoparticles into 3-dimensional networks. These hierarchical structures have been used for various optics and sensor applications. In general, semiconducting or metallic nanocrystals or monodisperse polymeric beads are the building blocks for this “bottom up” synthetic approach, where the structures are held together by van der Waals forces, hydrogen bonding, or other specific chemical interactions.